A diagnostic ultrasound system transmits ultrasonic waves to the inside of a subject by an ultrasound probe, receives a reflected echo signal of the ultrasonic waves corresponding to the structure of the body tissue from the inside of the subject, reconstructs a cross-sectional image, such as a B-mode image, and displays this for diagnosis.
Recently, it has been proposed to measure ultrasound image data by applying compression a subject by a manual or mechanical method, determine the displacement of various body regions caused by the compression on the basis of frame data of two ultrasound images having different measurement time periods, and generate an elastic image representing the elasticity of the body tissue on the basis of the displacement data. Various physical values correlating to the elasticity of body tissue are known, and, for example, distortion or an elasticity modulus of the body tissue is used. Here, distortion is a relative value obtained by spatially differentiating displacement, which is the amount of movement of the body tissue, and elasticity is a quantitative value obtained by dividing the change in stress acting on each body region by displacement.
For an elastic image, for example, as discussed in Patent Document 1, an elastic image colorized by adding color information, such as red, blue, and other colors, to regions in the tissue in accordance with the distortion or the elasticity modulus.
Since the displacement of the body tissue changes depending on the magnitude of the compression force, even for body tissue in the same region, distortion increases when strongly compressed. Therefore, a color elastic image representing distortion merely represents a relative display of the distortion of the regions on the color elastic image, and hardness cannot be quantitatively evaluated.
Patent Document 2 proposes to set two regions of interest (ROIs) in a B-mode image, measure the distortions in the two ROIs, and displaying numerical values of distortions in relation to the two ROIs on an image. Accordingly, by comparing the distortions of the two ROIs, the hardness of the ROIs can be relatively evaluated.
In other words, according to the technique described in Patent Document 2, the two ROIs may be set to a region that is presumed to be a tumor, such as cancer, and a region where no tumor exist, the distortions in the two ROIs are compared, and the distortion of the tumor is relatively evaluated with respect to that of the region without a tumor.
However, since a B-mode image does not represent distortion of tissue, it is difficult to recognize regions having different distortions. Thus, according to the technique described in Patent Document 2, it is difficult to appropriately set the magnitudes and the positions of the two ROIs whose distortions are to be compared.
Moreover, according to Patent Document 2, hardness cannot be quantitatively evaluated because consideration is not taken into account for a case in which, for example, the hardness of a region that is presumed to be an affected region, such as a tumor, be represented as an index, such as a ratio, with reference to a region of normal tissue or a region of body tissue having less individual difference in hardness.
Patent Document 1: JP2000-60853A
Patent Document 2: US2002/017883A1